The present invention is directed, in general, to an apparatus and method for detecting very low frequency acoustic signals and, more specifically, to an apparatus and method for monitoring physiological conditions such as respiration and cardiac activity, and other types of physiological information utilizing very low frequency acoustic signals. The present invention is directed to an apparatus and method for detecting very low frequency acoustic signals that represent physiological activity. The present invention comprises a sensor and a method for detecting very low frequency acoustic signals in the frequency range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz). The sensor of the present invention is capable of obtaining physiological condition signals from a person without being directly coupled to the skin of the person.
Microphones in physiological condition monitors are used to detect sounds that are indicative of physiological processes. Physiological condition monitors are capable of obtaining and recording signals indicative of a person""s physiological processes. The most commonly monitored physiological processes are respiration and cardiac activity. Physiological condition monitors that monitor respiration and cardiac activity usually comprise one or more sensors coupled to the body of the person whose physiological conditions are to be measured. The sensors are capable of sensing changes in physical parameters that are caused by the person""s respiration and cardiac activity. Physiological condition monitors measure and record waveform signals received from the sensors. Electrocardiogram (ECG) waveform signals are the most commonly used waveforms for measuring a person""s cardiac activity. Respiration waveform signals may be electronically derived using techniques such as impedance pneumography or inductive plethysmography. Respiration waveform signals are used to measure a person""s breathing rate and other types of information concerning respiration.
The present invention comprises a chamber and a microphone that is capable of detecting very low frequency acoustic signals.
The present invention is capable of monitoring physiological conditions utilizing very low frequency acoustic signals. For purposes of illustration, the present invention will be described with reference to physiological condition monitors that are capable of monitoring respiration and cardiac activity. It is understood, however, that the present invention is not limited to use in respiration monitors, and is not limited to use in cardiac activity monitors, and is not limited to use in physiological condition monitors in general. The present invention may be used to detect, measure and record any type of very low frequency acoustic signal.
Low heart rate is referred to as bradycardia. High heart rate is referred to as tachycardia. Cessation of respiration is referred to as apnea. When a person exhibits apnea, bradycardia or tachycardia a life threatening condition very likely exists. Physiological condition monitors that are capable of continuously monitoring a person""s respiration and cardiac activity are extremely useful for quickly detecting apnea, bradycardia or tachycardia. Such physiological condition monitors are also useful for quickly detecting other abnormal conditions such as a very slow breathing rate or a very high breathing rate.
Infants who are susceptible to sudden infant death syndrome are known to exhibit apnea and bradycardia. Physiological condition monitors that are capable of continually monitoring respiration and cardiac activity are particularly useful in the early detection of apnea or bradycardia in infants. Most physiological condition monitors are equipped with an alarm system to sound an alert when such conditions are detected.
A physiological condition monitor may be coupled directly to a person who is a patient in a hospital bed. In such an arrangement the waveform signals from the sensors coupled to the patient""s body may be sent through wires directly to a detector circuit (and other circuitry) located in a console by the patient""s bed. The wires attached to the patient restrict the patient""s movements and frequently become tangled as the patient moves. The tangling of the wires can also result in the sensors becoming detached from the patient. The loss of sensor contact can set off an alarm signal.
In other cases it is more practical to provide one or more sensors located in a belt, harness or item of clothing that is to be worn by the person to be monitored. In this type of physiological condition monitor the waveform signal information from the sensors is transmitted via a radio frequency transmitter to a radio frequency receiver in a base station unit that is located away from the site of the physiological condition sensors. The base station unit contains circuitry for analyzing and recording the waveform signal information. The base station unit contains circuitry for detecting abnormal conditions in the person""s breathing (such as apnea) or abnormal conditions in the person""s cardiac activity (such as bradycardia or tachycardia). Because of the freedom of movement that this type of monitor provides, it is the preferred type of monitor for monitoring the physiological conditions of infants.
If the data that is acquired by the physiological condition monitor is not transmitted to the base station unit and recorded there, then the data may be recorded in a memory data storage device located within the physiological condition monitor. To preserve the freedom of movement that is provided by a monitor that is worn on a belt, harness or item of clothing, the memory data storage device within the physiological condition monitor must be battery powered.
Electrocardiogram (ECG) waveform signals are commonly used to obtain information concerning a person""s cardiac activity. To obtain ECG waveforms an ECG sensor unit is coupled to the person whose cardiac activity is to be measured. The ECG sensor unit is coupled to the person via electrodes capable of receiving cardiac activity signals directly from the person""s body. In such an arrangement the electrodes must be attached directly to the person""s skin in order to receive the signals. The ECG sensor unit receives the ECG electrical signals from the electrodes. The ECG signals received by the ECG sensor unit are then either recorded within the physiological condition monitor or transmitted to a base station unit.
It is possible to obtain information about cardiac activity from acoustic signals. For example, U.S. Pat. No. 4,306,567 to Krasner discloses a sensor apparatus coupled directly to the skin of a person. The Krasner sensor apparatus is capable of detecting acoustic signals from cardiac contractions within a frequency bandwidth between about thirty Hertz (30.0 Hz) and ninety Hertz (90.0 Hz). The acoustical energy associated with the cardiac contractions detected by the Krasner sensor apparatus exhibits a maximum signal-to-noise ratio at about forty five Hertz (45.0 Hz).
The Krasner sensor apparatus is also capable of detecting acoustic signals from breathing activity within a frequency bandwidth between about three hundred Hertz (300.0 Hz) and six hundred Hertz (600.0 Hz). The acoustical energy associated with the breathing activity detected by the Krasner sensor exhibits a maximum signal-to-noise ratio at about four hundred Hertz (400.0 Hz). The Krasner sensor simultaneously detects both the cardiac activity signals at about forty five Hertz (45.0 Hz) and the breathing activity signals at about four hundred Hertz (400.0 Hz) with a single sensor unit coupled directly to the skin.
Acoustic signals normally contain noise artifacts. We have determined that most of the noise artifacts present in acoustic signals due to respiration and cardiac activity may be eliminated by considering only the very low frequency components of acoustic signals. In particular, almost all noise artifacts that are present in acoustic signals that are due to respiration and cardiac activity may be totally eliminated by filtering out all components of the signal that are outside the frequency range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz). This is due to the fact that most noise artifacts occur at frequencies that are higher than these frequencies.
We have also determined that sensor devices capable of detecting signals in the very low acoustic frequency range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz) do not need to be coupled directly to the skin of the person whose physiological conditions are being monitored. A sensor device that detects acoustic signals in the very low acoustic frequency range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz) in accordance with the principles of the present invention is capable of detecting indirect acoustic signals from the body of the monitored person through the monitored person""s clothes.
For these reasons it is advantageous to be able to detect very low frequency acoustic signals in the range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz). It is also advantageous to have an apparatus for monitoring physiological conditions in which it is not necessary to couple a sensor unit directly to the skin of the person to be monitored. It is also advantageous to have an apparatus for monitoring physiological conditions which is capable of detecting acoustic signals through the monitored person""s clothes.
The present invention comprises an improved apparatus and method for detecting very low frequency acoustic signals in the range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz). The very low frequency acoustic signals are useful in monitoring physiological conditions such as respiration and cardiac activity. The present invention is capable of detecting signals in a frequency range that is lower than the range of frequencies previously used to detect acoustic signals for monitoring physiological conditions.
An advantageous embodiment of the present invention comprises a chamber and a microphone that is capable of detecting very low frequency acoustic signals in the range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz). An advantageous embodiment of the chamber of the present invention comprises a closed chamber containing a fluid. The fluid may be either a liquid or a gas. In most instances the fluid that is used is air. The walls of the chamber are not completely rigid. The walls of the chamber are capable of expanding and contracting (i.e., moving inwardly and outwardly with respect to the interior cavity of the chamber) in response to external inputs of mechanical energy that form waves of very low frequency acoustical energy within the chamber.
The mechanical energy from outside the chamber forms waves of very low frequency acoustical energy within the chamber and causes the walls of the chamber to expand and contract by extremely small amounts. The extremely small expansions and contractions of the walls of the chamber cause the molecules of fluid in the chamber (usually molecules of air) to move in low frequency acoustic waves throughout the cavity of the chamber.
The present invention further comprises a microphone within the chamber. The microphone is capable of detecting the low frequency acoustic waves of the molecules of fluid in the chamber that are caused by the mechanical energy that causes the walls of the chamber to expand and contract.
Prior art acoustic sensors directly detect higher frequency sounds that are made by the lungs during respiration or by the heart during cardiac activity. The sensor of the present invention, however, obtains information by detecting very low frequency signals caused by the motion of the chest during respiration and by detecting very low frequency signals associated with cardiac activity. Almost all of the noise components in an acoustic signal have frequencies that are above the very low frequency range. By using the method of the present invention to exclude the higher frequencies of sound (and noise), the sensor of the present invention eliminates almost all the noise artifacts from the acoustic signal.
The present invention is capable of detecting acoustic signals from cardiac activity within a frequency bandwidth between about ten Hertz (10.0 Hz) and thirty Hertz (30.0 Hz). The acoustical energy associated with the cardiac activity detected by the present invention exhibits a maximum signal-to-noise ratio at about sixteen Hertz (16.0 Hz).
The present invention is capable of detecting acoustic signals from respiration within a frequency bandwidth between about one tenth Hertz (0.1 Hz) and two Hertz (2.0 Hz). The acoustical energy associated with the respiration detected by the present invention exhibits a maximum signal-to-noise ratio at about one and one half Hertz (1.5 Hz).
It is a primary object of the present invention to provide an improved apparatus and method for detecting very low frequency acoustic signals in the frequency range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz).
It is also an object of the present invention to provide an improved physiological condition monitor capable of detecting very low frequency acoustic signals in the frequency range of one tenth Hertz (0.1 Hz) to thirty Hertz (30.0 Hz) indicative of physiological conditions.
It is also an object of the present invention to provide an improved physiological condition monitor with a sensor unit capable of detecting very low frequency acoustic signals indicative of physiological conditions where the sensor unit is not coupled directly to the skin of the person being monitored.
It is also an object of the present invention to provide an improved physiological condition monitor with a sensor unit capable of detecting very low frequency acoustic signals indicative of physiological conditions where the sensor unit is capable of detecting such signals through the clothes of the person being monitored.
It is also an object of the present invention to provide an improved physiological condition monitor capable of detecting acoustic signals from cardiac activity within a frequency bandwidth between about ten Hertz (10.0 Hz) and thirty Hertz (30.0 Hz).
It is a further object of the present invention to provide an improved physiological condition monitor capable of detecting acoustic signals from respiration within a frequency bandwidth between about one tenth Hertz (0.1 Hz) and two Hertz (2.0 Hz).
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the Detailed Description, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprisexe2x80x9d and derivatives thereof mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9ceassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware, or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document. Those of ordinary skill in the art should understand that in many, if not most, instances, such definitions apply to prior, as well as future uses of such defined words and phrases.